The Inﬂuence of Magnetospheric Substorms on SuperDARN...

[email protected] Fall AGU 2007

The Influence of Magnetospheric Substorms on SuperDARN Backscatter

J.A. Wild1 & A. Grocott2

1. Space Plasma Environment and Radio Science Group, Dept. of Communication Systems, Lancaster University, UK.

2. Radio & Space Plasma Physics Group, Dept. of Physics & Astronomy, University of Leicester, UK.

Moonlight and aurora captured by the new Rainbow ASI at the Pykkvibær SuperDARN radar, Iceland on 20 Nov 2007.

SuperDARN Workshop 2008

SuperDARN A network of 19 coherent-scatter HF radars

In order to detect backscatter…• Irregularities must exist• Radar signal must propagate to/from irregularities• Signals must be orthogonal to irregularities


BACKGROUND & MOTIVATIONSubstorm Onset

From Lewis et al., 1997.


IDENTIFYING SUBSTORMS: IMAGEPrime data source were the WIC images (SI-13 images were used when WIC data were unavailable).

• a clear local brightening of the aurora has to occur

• the aurora has to expand to the poleward boundary of the auroral oval and spread azimuthally in local time for at least 20 min

• a substorm onset was only accepted as a separate event if at least 30 min had passed after the previous onset


SUBSTORM DATABASE

Frey et al. (2004)IMAGE WIC May 2000- Dec 2002

2437 substorms

IMAGE WIC May 2000- Dec 2005

4193 substorms

Exclude events within ±2 hours of another event 3005 substorms











Data are gridded in cells• 1° in latitude (≈111 km)• ≈111 km longitude

Same as SuperDARN “potential-mapping” technique

Gridded data span 2 min intervals ±90 min from onset

No spatial averaging

No temporal averaging

Ground scatter excluded

Noise removed


Data are gridded in cells• 1° in latitude (≈111 km)• ≈111 km longitude

Same as SuperDARN “potential-mapping” technique

Gridded data span 2 min intervals ±90 min from onset

No spatial averaging

No temporal averaging

Ground scatter excluded

Noise removed


€

Ψ(t) =nscatter(t)nradars(t)

Compute a backscatter parameter:

€

Ψ =1817

= 25.86

SuperDARN Workshop 2008€


BACKSCATTER VARIATIONS DURING SUBSTORMS



BACKSCATTER VARIATIONS DURING SUBSTORMS

[email protected] Fall AGU 2007

SPATIAL BACKSCATTER VARIATIONS

Ψ Ψ























-30 0



Ψ(t,ν ) =nscatter(t,ν )nradars(t,ν)

BACKSCATTER-FREQUENCY BEHAVIOUR

21-03 MLT60°-70° Mlat


Ψ(t,ν ) =nscatter(t,ν )nradars(t,ν)

BACKSCATTER-FREQUENCY BEHAVIOUR

21-03 MLT70°-80° Mlat


FINDINGS• NH radars observe approx twice as much

backscatter as SH radar

From Milan et al., 1997.


FINDINGS• Globally, the amount of

backscatter observed by SuperDARN peaks a few minutes prior to expansion phase onset

• In the nightside ionosphere:•Scatter falls overall•Reduction at 70°- 80° Mlat• Increases in at 60° - 70° Mlat•Equatorward motion of backscatter

• Possible to use “stereo” developments of SuperDARN system to maximise scatter at different latitudes?


FUTURE DEVELOPMENTS

The influence of magnetospheric substorms on

SuperDARN radar backscatter

J. A. Wild1 and A. Grocott2

Received 24 October 2007; revised 3 January 2008; accepted 5 February 2008; published 25 April 2008.

[1] The SuperDARN ionospheric radar network is a leading tool for investigating thenear-Earth space environment. However, reductions in ionospheric backscatter have beenreported during magnetospheric substorms. We have therefore investigated the impactof substorms upon SuperDARN backscatter during 3005 substorms and find that theglobal level of scatter maximizes just prior to substorm onset. In the nightside ionosphere,backscatter poleward of !70! magnetic latitude is reduced, with radar echoes shiftingto lower latitudes. An examination into the frequency-dependence of nightside backscatterevolution during substorms reveals that although most backscatter data is based uponoperations in the 08–14 MHz range, higher operating frequencies may offer improvedperformance in the period just prior to and immediately following expansion phase onset.We suggest that the SuperDARN array of high-frequency coherent-scatter radars, andin particular those radars with the ability to simultaneously operate at dual frequencies,will play a key role in future space- and ground-based studies of substorms.

Citation: Wild, J. A., and A. Grocott (2008), The influence of magnetospheric substorms on SuperDARN radar backscatter,J. Geophys. Res., 113, A04308, doi:10.1029/2007JA012910.

1. Introduction

[2] Since the concept was first proposed by Akasofu[1964], the substorm has proven to be one of the greatestchallenges in solar-terrestrial physics. Despite advances inthe field, the timing, location and possible triggering mech-anism of substorm onset remains unclear, with competingmodels seeking to explain the instability underlying theexplosive reconfiguration during the substorm expansionphase [e.g., Lui, 2003].[3] The Super Dual Auroral Radar Network (SuperDARN:

Chisham et al. [2007]) is an international array of 18 high-frequency (HF) coherent-scatter ionospheric radars withfields-of-view covering a significant fraction of the auroraland polar ionosphere in both the northern and southernhemispheres. Data from a subset of the network can beanalyzed to provide detailed localized measurements ofionospheric plasma dynamics while measurements fromall radars may be combined using the ‘‘potential mapping’’technique of Ruohoniemi and Baker [1998] in order toestimate the global ionospheric convection pattern in bothhemispheres. Consequently, SuperDARN has become oneof the pre-eminent ground-based tools for the investigationof the space and ionospheric plasma environment and a vitaltool when undertaking combined space- and ground-basedinvestigations [e.g., Amm et al., 2005].

[4] The SuperDARN system has provided significantinroads to the substorm problem by revealing ionosphericflows in the nightside ionosphere during both the growthand expansion phase, the response of the ionosphericconvection pattern to the increased tail reconnection rateduring the expansion phase and the family of substorm-associated convection transients observable in the nightsideionosphere (the reader is directed to Chisham et al. [2007,section 5], for a comprehensive review). However, anequatorward migration of radar backscatter has previouslybeen reported during the substorm growth phase [Lewis etal., 1997] while a loss of backscatter (upon which allSuperDARN data products depend) is sometimes reportedin the nightside ionosphere during substorm onset, an effectattributed to absorption of HF radio waves by the enhancedelectron densities in the substorm precipitation region[Milan et al., 1999] and rapid changes in HF propagationconditions [Gauld et al., 2002].[5] Apart from case-studies of individual substorms, the

only previous study to examine the impact of magneto-spheric substorms upon SuperDARN radar backscatter wasthat of Provan et al. [2004]. In that study, SuperDARN datawas used to examine the northern hemisphere ionosphericconvection pattern during 67 substorms identified by the farultra violet (FUV) auroral imager on board the IMAGEsatellite. Provan and coworkers reported little change in theoccurrence of radar backscatter during the substorm growthphase with the highest number of radar echoes observed inthe post-noon sector dayside ionosphere. Following sub-storm onset, this post-noon sector backscatter grew strongerwhile nightside scatter diminished and showed some evi-dence of equatorward migration.

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, A04308, doi:10.1029/2007JA012910, 2008ClickHere

for

FullArticle

1Department of Communication Systems, Lancaster University, Lancaster,UK.

2Department of Physics and Astronomy, University of Leicester,Leicester, UK.

Copyright 2008 by the American Geophysical Union.0148-0227/08/2007JA012910$09.00

A04308 1 of 6

• This work published recently“The Influence of Magnetospheric Substorms on SuperDARN Radar Backscatter”Wild & Grocott, JGR, 2008.

• Follow on work looking at flows“The influence of Magnetospheric Substorms on High-Latitude Ionospheric Convection”Grocott, Wild, Milan & Yeoman• Poster presented at this meeting• Submission expected shortly

• Coming soon…Large scale analysis of SuperDARN Doppler spectral width during these 3005 substorms and comparison with IMAGE WIC optical data.

Refs

.

Conclusions

Low-latitude substorms: are generally of larger intensity and are associated with intervals of stronger convection, BUT more noticeably suppress the flow immediately after onset

Mid-latitude substorms: have a more significant effect globally than high-latitude substorms but do not produce a very large enhancement in the flows locally

High-latitude substorms: are slower at producing a large-scale convection response but produce the most noticeable enhancement to the flow in the locally disturbed region

LOW

MED

IUM

H

IGH

Radio and Space Plasma Physics Group

The influence of magnetospheric substorms on high-latitude ionospheric convection

Adrian Grocott1, Jim Wild2, Steve Milan1, Tim Yeoman11 University of Leicester; 2 Lancaster University

Why do we want to know?

Substorms are a global process

THEMIS will make unprecedented in-situ observations but these will still be local point

measurements

The high-latitude ionosphere can tell us about the dynamics of the entire magnetosphere

Introduction

A number of statistical studies have attempted to determine the ionospheric convection response to substorms (e.g. Provan et al., 2004; Bristow and Jensen, 2007)

These studies have involved a limited number of substorms such that all events had to be artificially combined into a single substorm coordinate system

Here we analyse SuperDARN radar data from 1979 northern hemisphere isolated substorms that were identified in IMAGE FUV satellite data (Frey et al., 2004; Wild and Grocott, 2008)

The substorms have then been grouped according to onset latitude using similar criteria to Milan et al. (2008) in their discussion of average substorm auroral evolution

The local and global influence of substorms on the average SuperDARN convection patterns has then been studied

Bristow and Jensen, A superposed epoch study of convection during substorms, J. Geophys. Res., 112, 2007.Frey et al., Substorm onset observations by IMAGE-FUV, J. Geophys. Res., 109, 2004.Milan et al., A superposed epoch analysis of auroral evolution during substorms, ICS-9, 2008.Provan et al., Statistical study of high-latitude plasma flow during substorms, Ann. Geophys., 22, 2004.Wild and Grocott, The Influence of Substorms on SuperDARN Backscatter, J. Geophys. Res, in press, 2008.

Large-scale convection be-

comes enhanced during the growth

phase (due to dayside

reconnection)

Lower-latitude substorms are

associated with more intense pre-

existing convection

By ~20 minutes into the expansion phase all latitude bins show an enhancement to the nightside convection

The suppression of flow at substorm onset is most evident for low-latitude events

After ~80 minutes the flows related to high latitude substorms are subsiding whereas those associated with low latitude ones remain intense

The Harangdiscontinuity is most evident for mid-latitude substorms

HIGH

MEDIUM

LOW

Growth Phase Expansion / Recovery PhaseOnset

Sub

sto

rm S

tatis

tics

SuperDARN Average Substorm Convection Maps

Low-latitude: strongest overall convection but most severe post-onset drop

Mid-latitude: modest convection enhancement during the expansion phase

High-latitude: show a marked convection enhancement which begins ~20 minutes post-onset

HIG

HM

EDIU

MLO

W

Global Response

• The fastest flows are in the dusk convection cell• The nightside flows are in general the slowest• There is a definite enhancement in the nightside

flows for high latitude onset events• The enhancement is less for medium latitude onset

events with a decrease evident for low-latitude onset events

HIGH MEDIUM LOW

email: [email protected]

0.7 hoffset

Substorm onset MLT is only weakly correlated to IMF clock angle

Loc

al

Resp

ons

e

Substorm onset latitude is correlated to both IMF clock angle and substorm intensity

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The Inﬂuence of Magnetospheric Substorms on SuperDARN...

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